Also are you using suPHP please refer to the following page for suPHP configuration :
http://softaculous.com/docs/SuPHP_Settings and if you are not using suPHP the above fields should be blank.
Sir you were facing the internal server error because you had not configured the suPHP settings. And after configuring the settings the script was dying because mcrypt is not installed on your server. If you install mcrypt on your server PrestaShop should work fine.